@campbellbarton: This operator works (as intended) on an object level, wich means that it won't remove doubles for vertices that are close to each other but contained in different objects - is that really helpful?

Brecht authored this commit, but he gave me the honours to actually
do it. Here it goes; Blender Internal. Bye bye, you did great!

* Point density, voxel data, ocean, environment map textures were removed,
as these only worked within BI rendering. Note that the ocean modifier
and the Cycles point density shader node continue to work.
* Dynamic paint using material shading was removed, as this only worked
with BI. If we ever wanted to support this again probably it should go
through the baking API.
* GPU shader export through the Python API was removed. This only worked
for the old BI GLSL shaders, which no longer exists. Doing something
similar for Eevee would be significantly more complicated because it
uses a lot of multiplass rendering and logic outside the shader, it's
probably impractical.
* Collada material import / export code is mostly gone, as it only worked
for BI materials. We need to add Cycles / Eevee material support at some
point.
* The mesh noise operator was removed since it only worked with BI
material texture slots. A displacement modifier can be used instead.
* The delete texture paint slot operator was removed since it only worked
for BI material texture slots. Could be added back with node support.

* Not all legacy viewport features are supported in the new viewport, but
their code was removed. If we need to bring anything back we can look at
older git revisions.
* There is some legacy viewport code that I could not remove yet, and some
that I probably missed.
* Shader node execution code was left mostly intact, even though it is not
used anywhere now. We may eventually use this to replace the texture
nodes with Cycles / Eevee shader nodes.

* The Cycles Bake panel now includes settings for baking multires normal
and displacement maps. The underlying code needs to be merged properly,
and we plan to add back support for multires AO baking and add support
to Cycles baking for features like vertex color, displacement, and other
missing baking features.

* This commit removes DNA and the Python API for BI material, lamp, world
and scene settings. This breaks a lot of addons.
* There is more DNA that can be removed or renamed, where Cycles or Eevee
are reusing some old BI properties but the names are not really correct
anymore.
* Texture slots for materials, lamps and world were removed. They remain
for brushes, particles and freestyle linestyles.
* 'BLENDER_RENDER' remains in the COMPAT_ENGINES of UI panels. Cycles and
other renderers use this to find all panels to show, minus a few panels
that they have their own replacement for.

The work is mainly from Lukas Toenne, with some modifications from myself.

Includes following obvious changes:

- Particle system selection is now name-based, with lookup menu.
- Lots of new options to control varieties.

Changes comparing to the Gooseberry branch:

- Default values and versioning code ensures same behavior as the
old modifier.

- Custom data layers are coming from vertex color, the modifier
does not create arbitrary layers now. The hope is to keep data
more manageable, and maybe make it easier to select in the shader
later on.

This means, values are quantized to 256 values, but it should be
enough to get varieties in practice.

- added `object_color_type` where the user can set if the collection
determines the color, or the object will be used for the color.
Implemented it as an enum as later this can have a random color option.
- moved OB_LIGHTING_* to DNA_view3d_types and renamed it.
- Fixed some DRY in workbench_materials.c. Can remove more DRY's but
will need to discuss the responsibility of the workbench engine as it
might become part of the eevee renderer.

ViewRender was removed, which means we can't get the render engine for files
saved in 2.8. We assume that any files saved in 2.8 were intended to use Eevee
and set the engine to that.

A fix included with this is that .blend thumbails now draw with Clay mode,
and never Eevee or Cycles. These were drawn with solid mode in 2.7, and should
be very fast and not e.g. load heavy image textures.

Assume files saved in 2.8 were intended for Eevee and set them to material
viewport shading. In Eevee this is equal to rendered draw mode, in Cycles
this will draw with Eevee. This way Eevee demo files still show something
interesting when opened.

This statement is only relevant in 2.8, but causes confusion in master.
I kept the 'default' label to prevent compiler warnings about unhandled
cases. The break is needed because there should be at least one statement
after 'default'.

I.E. only enable auto-override for 'active' selected object when making
an override of a linked group. This will ease on auto-override creation,
and you typically do not want to auto-override most objects in the group
anyway (in proxy system, you could only proxyfy one object of the group
anyaway!).

This finally allows us to use Random factor to add variations to the
interpolated children. This feature never worked since 2007L there was
random factor slider in the interface, but it was only used by simple
children. Now it has affect on interpolated children as well.

Technically, this will break compatibility if older file had random
factor set to something else than 0 (default value is 0 though). But
we are leaving 2.7 series, so can accept such breackage in the name
of supported features.